The present invention relates to a process for preparing polymers of alpha-methylstyrene. More particularly, the present invention relates to an improved process allowing for the preparation of highly pure polymers comprising in polymerized form alpha-methylstyrene and a copolymerizable vinyl aromatic comonomer.
It is previously known in the art that polymers of alpha-methylstyrene may be readily prepared by anionic polymerization. A suitable process for preparing such polymers is disclosed, for example, in EP No. 87,165.
Polymers of alpha-methylstyrene and, in particular, copolymers of alpha-methylstyrene and copolymerizable vinyl aromatic comonomers, especially styrene, are highly desirable due to their relatively high glass transition temperatures and ease of fabrication. In certain end use applications such as in glazing, optical devices, and light transmission devices such as fiber optical cables, it is additionally desirable that the polymers exhibit good clarity and an absence of color.
Presently available anionic polymerization initiators such as the well known organo lithiums, have not allowed the attainment of highly pure alpha-methylstyrene polymers. In practice, relatively large amounts of such organolithium initiators, i.e. on the order of 500 parts per million based on polymer weight or greater are generally employed. In applications requiring improved clarity and reduced color in the resulting alpha-methylstyrene polymer, the use of these elevated amounts of organolithium initiator have been found to be unsatisfactory due to the level of lithium salts remaining in the finished polymer. Polymers for optical usages may be quantitatively evaluated for purity by measuring color, especially the yellowness thereof, according to published techniques, such as American Society of Testing Method Test ASTM-D1925. It has not previously been known to prepare polymers of alpha-methylstyrene having a yellowness index of 4.0 or less measured by the above ASTM-D1925 procedure.
In U.S. Pat. No. 3,356,754, it is disclosed that homopolymers of conjugated dienes or copolymers thereof with another conjugated diene or a vinyl substituted aromatic hydrocarbon may be prepared employing an initiator comprising an organolithium compound and a sodium, potassium, rubidium or cesium salt of an organo compound. The reference specifically excluded the preparation of alpha-methylstyrene copolymers. At Column 5, line 35, it was stated "it is to be understood that a compound having a substituent on the alpha-carbon atoms, such as alpha-methylstyrene, is not applicable to the practice of the instant invention."
In U.S. Pat. No. 4,246,372, later researchers reiterated this finding. In this patent, it was disclosed that block copolymers of alpha-methylstyrene and butadiene could not successfully be prepared in the presence of alkyllithium initiators and initiation activators such as potassium, rubidium or cesium alkoxides. Suitable polymers could be prepared only by conducting polymerization at reduced temperatures on the order of about 30.degree. C. and subsequently capping the prepared polymer with polybutadiene. At elevated temperatures only random copolymers of alpha-methylstyrene and butadiene could be prepared.
It would be desirable to provide an improved process for the anionic polymerization of alpha-methylstyrene.
It would be further desirable to provide a process for the anionic polymerization of alpha-methylstyrene at elevated temperatures that does not require the presence of a conjugated diene to provide capping of the living polymer.
It would be desirable to provide an anionic polymerization process for the preparation of copolymers of alpha-methylstyrene and a copolymerizable vinyl aromatic comonomer employing reduced levels of organo lithium initiator in order to provide copolymers thereof having excellent purity, clarity and color.